R problems with lapack with gfortran

On Sat, May 04, 2019 at 06:42:47PM +0200, Thomas K?nig wrote:

- figure out Fortran2003 specification for C/Fortran interoperability
-- this _sounds_ like the right solution, but I don't think many
understand how to use it and what is implied (in particular, will
it require making changes to LAPACK itself?)

That would actually be fairly easy.  If you declare the subroutines
BIND(C), as in

        subroutine foo(a,b) BIND(C,name="foo_")
        real a
        character*1 b
        end

you will get the calling signature that you already have in your C
sources.

This also has the advantage of being standards compliant, and would be
probably be the preferred method.

With the caveat that one may need to use the VALUE attribute to
account for pass-by-value vs pass-by-reference.

This seems clean solution, but as I said before not easy, because 
currently the tradition is to call the Fortran interface directly from C 
(not via any C wrappers). This means one could not substitute 
LAPACK/BLAS at dynamic linking time, unless all LAPACK/BLAS 
implementations agreed on such a C interface. Now the substitution is 
based on the original Fortran interface.

In case of R, if we only used the included reference BLAS/LAPACK, we 
could do this, define our wrappers, say "c_dgemm" for "dgemm", change R 
to call via that interface, ask maintainers of all packages to change 
their code to call via their interface, and this should work with all 
Fortran 2003 compilers.

But, R is often used also with optimized BLAS/LAPACK implementations 
that can be substituted at dynamic linking time. And there we could do 
nothing at R level to help: we cannot generate such wrappers for an 
existing LAPACK/BLAS implementation (we don't have the source code, the 
compiler, etc).

It would be certainly a good thing if BLAS/LAPACK, with all 
implementation and uses, switched to a way that is compliant with 
current Fortran standard. But this should best start with the reference 
BLAS/LAPACK, continue with other BLAS/LAPACK implementations, and then 
with systems using those libraries, including R and its packages. 
Unless/before this happens, it would really be great if we could still 
use gfortran to build and use this fundamental software library.

Best
Tomas